Configuration of Cycles fisheye camera
The angle of coverage of a lens is determined by the size of the sensor and the focal length.

Blender is designed to deal with variation of all of these parameters as they would happen in real life in a real camera.

When you change the sensor size, the Angle of Coverage of a lens will be affected.

As an example. A 35mm lens in a full sensor 35mm camera with a sensor that is 36mm wide would be considered a "normal" lens (close to that of the human vision) with an angle of coverage of 54.432 degrees. 

The same 50mm lens on a sensor that is 8.8mm Horizontally would be a very long telephoto with a field of view of barely 14.331 degrees.

The same lens on a large form at camera with a sensor with of 6.5cm would be a wide angle lens with a field of view of 89 degrees